1. Field of the Invention
The present invention relates to a process for fabricating superconducting tape, and more particularly, the fabricated superconducting tape is highly flexible and nonetheless retains high critical current density when it is curved with a small radius of curvature at the temperature of liquid nitrogen (77.degree. K.).
2. Description of Prior Art
To put superconducting material into practical or commercial use, not only the superconducting properties thereof should be considered, i.e. the critical temperature T.sub.c, the critical magnetic field H.sub.c, and the critical current density J.sub.c, but also the mechanical properties thereof such as the flexibility, the strength, the hardness, and the ductility etc. Among these mechanical properties, ductility and flexibility are generally considered the most important factors in determining the utility values of a superconducting material since electric conductors are often elongated and wound into coils for producing magnetic field. If a superconducting material is brittle, it is difficult to be made into electrical conducting wires and consequently has few utility values.
Accordingly, it is usually an important object for superconductor researchers to discover superconducting materials which not only provides superconducting properties at a certain temperature but also provides flexibility so as to be capable of being wound into a coil with a small diameter.
For practical applications, the critical current density J.sub.c of a superconducting wire should be as large as 10.sup.4 A/cm.sup.2 in order to produce usable high magnetic field. The superconducting wire is generally wound into coils for this purpose. However, in view of a known principle that a superconducting wire provides the largest critical current density J.sub.c when it is a straight line and as the superconducting wire is bent, the critical current density J.sub.c therein will be degenerated. The larger the curvature of the wire becomes, the more the critical current density J.sub.c degenerates. Accordingly, as the superconducting wire is wound into a coil with a smaller diameter, the magnetic field produced thereby will also be degenerated.
In a U.S. Pat. No. 4,975,416, Onishi et al teach a. method of producing superconducting ceramic wire. The patent discloses two chemical compositions produced by the method; the first composition is of the following form: EQU Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x,
and which is produced by using a mixture selected from the group consisting of Bi.sub.2 O.sub.3, SrO.sub.3, CaCO.sub.3, and CuO and is referred to as System A; and the second composition is of the following form: EQU Bi.sub.1.6 Pb.sub.0.4 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x
and which is produced by using a mixture selected from the group consisting of Bi.sub.2 O.sub.3, PbO, SrO.sub.3, CaCO.sub.3, and CuO and is referred to as System B.
A superconducting wire made of either System A or System B is capable of being bent into the shape of an arc with a radius of curvature down to about 0.5 cm but the critical current density J.sub.c thereof is only 250 A/cm.sup.2. The superconducting wire made of System B has a critical temperature Tc(R=0) 101.degree. K. (where R=0 herein and hereinafter represents zero resistance), and a critical current density J.sub.c =100 A/cm.sup.2 at 77.degree. K. in zero magnetic field. Furthermore, if Ag.sub.2 O is added to the composition of System B in an amount of 20% by weight thereof, two wires with superior flexibility can be produced, one with: EQU Tc(R=0)=87.degree. K., Jc=250A/cm.sup.2 (70.degree. K., zero magnetic field),
and the other with: EQU Tc(R=0)=102.degree. K., Jc=250 A/cm.sup.2 (77.degree. K., zero magnetic field).
The Cnishi et al patent has successfully disclosed a highly flexible superconducting wire. However, the critical current density J.sub.c is still not a satisfactory one when the wire is curved into a coil with a small radius of curvature.